Method for comparative studies of a metal{40 s corrosion resistance and apparatus as an aid to these investigations

ABSTRACT

For rapid information about the resistance to corrosion of a metal body or about the formation of a protective coating thereon, a number of test specimens are located in a stream of corrosive agent in such a manner that the flow velocity of the agent is different past each specimen. The specimens are in turn connected to an electric circuit in such a manner that they may be individually disconnected therefrom and connected to a measuring instrument which records the galvanic current between the specimen in question and the remaining specimens, the magnitude of the current being a function of the corrosive action at that particular specimen.

United States atem [54] METHOD FOR COMPARATIVE STUDIES OF A METAL'S CORROSION RESISTANCE AND APPARATUS AS AN AID TO THESE INVESTIGATIONS 7 Claims, 9 Drawing Figs. [52] U.S. Cl 23/230 C, 23/253 C, 73/86, 324/71 C [51] Int. Cl ..G0ln 17/00 [50] Field of Search 23/230 C, 253 C, 253, 230; 73/86, H; 324/71 C [56] References Cited UNITED STATES PATENTS 2,405,532 8/l946 Todd 23/253 C 2,374,088 4/1945 Fontana et al. 23/230 C X 2,824,283 2/l958 Ellison 23/230cx 2,897,060 7/1959 Dieman 23/2530 OTHER REFERENCES Champion, F. A., Corrosion Testing Procedures; John Urley & Sons, Inc., N.Y., 2nd edition; 1965 (pp. 282- 288 relied on) copy in Scientific Library.

Ulig, H. H., The Corrosion Handbook; John Urley & Sons; N.Y., I948 (page 985 relied on) copy in Art Unit 171.

Primary Examiner-Morris O. Wolk Assistant Examiner-Barry S. Richman Attorney-l-lolman & Stern ABSTRACT: For rapid information about the resistance to corrosion of a metal body or about the formation of a protective coating thereon, a number of test specimens are located in a stream of corrosive agent in such a manner that the flow velocity of the agent is different past each specimen. The specimens are in turn connected to an electric circuit in such a manner that they may be individually disconnected therefrom and connected to a measuring instrument which records the galvanic current between the specimen in question and the remaining specimens, the magnitude of the current being a function of the corrosive action at that particular specimen.

PAIENTEUunv 23 l97| 3, 622.274

sum 1 0F a METHOD FOR QOMPARATIVE STUDES OF A METAL S EORROSION RESKSTANCE AND APPARATUS AS AN AID TO TEESE INVESTEGA'IEIONS BACKGROUND OF THE INVENTION The invention concerns a method for comparative studies of the corrosion resistance of a metal to a certain corrosive agent and/or to a coating applied to the metal and its possibility of preventing corrosion, especially in such cases where the corrosive agent is in motion.

Up to now the most common method to determine the corrosion resistance of a metal has been to expose a number of test specimens for a long period of time to such conditions as can be expected to occur in normal use of the metal. Normally the test specimens are placed in the desired environment for a considerable length of time, e.g. a number of years. The result of the corrosion is mainly determined through weight and dimension losses of the specimens.

According to a common laboratory method a submerged test specimen is sprayed with a thin jet. This may result in an eroded hole, which in turn is studied for depth and appearance. Its evaluation forms a basis for empirical judgment of the corrosion resistance of the specimen. The method has been used for a long time and been adjusted primarily to testing materials for condenser tubes and harmonizes well with practical experience. The drawbacks with this method are that it calls for a great number of similar tests and the results have to be treated statistically due to the discrepancy in weight losses that are commonly experienced in different environments.

According to a similar method, a solid disk or cylinder of the desired material is rotated in a corrosive agent. If, for example, the metal is a copper alloy, the corrosion usually occurs at the outer part of the disk. By evaluating the depth of the attack and weight losses as against exposure time, the corrosion rate can be evaluated.

All these methods have in common that they have to be carried out for a relatively long period of time that, in some cases, runs into years. The results say little or nothing of the protective films forming or disintegrating on the surface being tested.

The screening efiect of a protective film that is essential for the materials speed of corrosion can however be determined through plotting a polarization diagram. However, in practice great difficulties are encountered in measuring the potentialdifference between the metal, the protective film and the fluid. Further, these measurements are time-consuming and therefore disturb the corrosion process. In reality what one measures is the potential-difference between fluid outside the protective film and the pure metal. This is especially true when a current load is included in the measurement as a potential-difference across the protective film (ion and electron resistance) is then included in the readings. Due to this fault in the measurements it is difficult to exactly define the screening effect of the protective film. A compact protective film has the efiect of quite considerably altering the potential from a balanced value when charged with a current load (steeper polarization curves). Usually a metal with a protective film shows higher values than a metal without such a protective film. When such metals are arranged according to their corrosion-potentials in, for example, sea water, one gets quite a different relative order as against if one had used normal potentials to decide the order.

From such a list it is only possible to determine in which direction galvanic currents will flow when difierent metals are connected. The list, however, gives no indication of the amount of the current and this has to be determined directly by means of a polarization diagram.

Potential measurements are sensitive to temperature and concentration differences, therefore great care should be ob served on comparing results from different testings. Potential measurements undertaken with respect to exposure time give an indication of possible formation of protective films, with the important reservation that no variation has occurred in the corrosive fluid during the time.

SUMMARY OF THE INVENTION The purpose of this invention is to propose a method and an apparatus whereby the corrosion resistance of a metal and the formation or destruction of a protective film thereon can be easily determined much faster than has been done previously. The method lends itself well to research into cavitation corrosion. The characteristics of the invention are primarily as follows: that the corrosive liquid is forced to flow past a number of insulated test specimens in such a way that each specimen is subjected to a different corrosive liquid velocities; that the test specimens during trials are short circuited and in sequential order are temporarily connected to a measuring instrument (Volt/Ampere-meter) for regular readings of galvanic currents between one specimen and the others still short circuited; and that the obtained readings are analyzed by plotting diagrams, where the short circuit current is plotted as a function of time, the size and nature of the short circuit current standing directly in relation to the severity of the corrosion attack, and the rate of possible film-formation and its density.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a longitudinal section through the inventive apparatus;

FIG. 2 shows a front view of one of the cable ends of the apparatus illustrating the location of the test specimens;

FIG. 3 shows schematically the internal electric connection between the test specimens and the recording instruments;

FIG. 4 shows on a larger scale a sectional view through one part of the cable end with the test specimens; and

FIGS. 5-9 show graphs of short circuit currents for an array of different metals.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method according to the invention can appear under difi'erent forms and the following description is therefore only one example, applied to an apparatus which works much like a pump, for the study of corrosion on copper-alloys, with sea water as a medium.

The apparatus according to the invention consists of a casing I made up of some noncorroding dielectric material, e.g. acryl-glass. The casing consists of a cylinder 2 arranged between two end walls 3 and 4 forming a chamber 5. An inlet 6 is arranged along the longitudinal axis of cylinder 2 in the outer end wall 3 while an outlet 7 is arranged in the wall of cylinder 2. In the inner end wall 4, opposite to the inlet 6, a shaft 8 is fitted, which is connected to a motor 9. The shaft 8 carries a rotor 10, which is located inside chamber 5 nearer to the end wall 3 than to the end wall 4, so that a wider clearance is formed between the rotor 10 and end wall 4 than between the rotor 10 and end wall 3. In this wider clearance a pitotstatic tune 11 is placed, forming the continuation of the outlet 7. The apparatus in this embodiment works like a pump, therefore no auxiliary pump is needed.

On the inside of the outer end wall three three series of seven identical test specimens l2al2g, l3a-I3g and 14a 14g are positioned as in FIG. 2. Each test specimen belonging to one series is machined from the same cast rod and is formed into a counter sunk bolt having a round heat 15 and threaded shaft 16, as best seen in FIG. 4. The test specimens are so positioned in the outer end wall 3, that the outer surface of the bolt head 15 is flush with the inner wall surface.

The bolt head 15 can be cylindrical or frustoconical, the hole 17 in the end wall 3 being shaped in a matching manner. A gasket 18 prevents leakage of the corrosive liquid. Each test specimen is by way of its shaft 16 in electrical connection with the other test specimens belonging to the same series, so that they are short circuited. The test specimens belonging to one series are so arranged as regards the inlet 6 that they are subjected to difierent corrosive liquid speeds. Preferably the test specimens are positioned along a radius in relation to the inlet. In order to be able to place many specimens in the apparatus, the positioning can also be arranged along helical lines as shown in FIG. 2. Regular recordings are made of the galvanic current between one test specimen and the other short circuited ones in the same series. The readings are either done visually with an amperemeter 219, or automatically with a registering instrument 2i).

By modifying the apparatus, cavitation corrosion can also be studied, and for this purpose a bore 21 is made: in front of each test specimen. The depth of this bore is adjustable by means of a plug 22, that can be formed like a screw.

The method according to the invention works as follows:

A corrosive agent, e.g. sea water, is introduced to the chamber 5 through the inlet 6 where it is subjected to severe stirring by the rotor W, whereafter the liquid leaves by means of the pitot-static tube 1'1 and the outlet 7. In this way the test specimens will be subjected to different water velocities depending on their distance from the center of the rotor. With a rotor speed of about 7250 r.p.m. the following flow speeds have been measured at the individual specimens of a series 9.0-I4.3-l9.4-24.7-30.0-35 and 40.0 m./s.

The heat generated by the rotor is controlled by controlling the admission and withdrawal of the corrosive agent, so that the temperature rise across the end wall 3 is 1C.

The corrosive agent-sea water-is continuously aerated and can be considered saturated thereby.

The test specimens are short circuited throughout the entire test, usually 230 hours, so that the seven specimens in each series can be considered as one unit. Thus the most important factors of a corrosion attack in a pump have been reproduced, namely, the realization at different fluid speeds of the shearing stress, the effect of varying turbulence, and galvanic currents between specimens subjected to difi'erent water speeds. Only when electrical readings are made is the short circuit broken for a short period of time. The recordings that have been regularly made have been of galvanic currents between each test specimen and the other short circuited test specimens in the same series and plots of these recordings are shown in FIGS. 5-9.

FIG. 5 shows short circuit currents plotted as against time for pure (99.90percent) copper. In this and in future diagrams, negative current means that the electrons flow from the metal, i.e. that electrons flow from test specimens sub jected to high flow velocities to the specimens in the same series subjected to smaller flow velocities. Height variations in these curves are due to temperature variations (see lower part of FIG. 5). The total increase of the current as against time is due to increased potential differences which have to do with the failure of the protective film formed on test specimens 0, b, c and d. For the first 20 some hours the current flow from the specimens subjected to high corrosive liquid velocity (e, f and g) decreased as the current caused the other specimens (0, b, c and d) to develop a protective film. It will be seen that in time the forming of the protective film stops forming and the current flow increases up to about 100th hour where it leveled out at rather high levels. Hence the films formed are poor.

FIGS. 6-9 show short circuit currents versus time for four different metal alloys. n commencement of the test the short circuit currents show the same direction and roughly the same numerical values as for pure copper (FIG. Towards the end of the test the alloys'have internally about the same current values, but with varying directions, i.e. electron flow is reversed. In FIG. 6 the specimens of the first alloy are subjected to the low corrosive liquid velocity (a, b and 0) receive protective current from d, e, f and g and build a protective film. After about 30 hours the specimens with the film (a, b and 0) no longer require the protective current and can supply current to specimens d, e, f and 3 so they can develop a protective film, hence the reversing in current flow. The specimens of the second alloy as seen in FIG. 7 act similar to the specimens of FIG. 6. It will be seen that after about the 40th hour the intensity of the currents varies only slightly with time. The short circuit current curves for the fourth alloy (FIG. 9) and the third alloy (HO. 8) rise sharply to a high maximum and thereafter fall back and stabilize at a relatively low value.

it will be seen in these Figures that specimen 3, after about the th hour, supplies all the other specimens with protective current, thereby causing rapid corrosion on specimen 3. it therefore is evident that the materials of FIGS. ti and 9 cannot be used where the corrosive liquid speeds are as high as those going past specimen 3.

All the specimens of the above-mentioned alloys are at the end of the test entirely covered with films: on the test pieces subjected to high corrosive liquid velocities and for some of the test pieces subjected to lower corrosive liquid velocities damage can be detected on the protective film.

Analysis of the weight losses through special tests indicate that the losses are encountered in the beginning before any protective film has had time to form.

All these tests compile to show that the magnitude, direction and general shape of the short circuit current curves are in direct relation to the corrosion damage and the mechanical properties of the possible protective film.

lfthe test specimens are in advance provided with an artificial protective it is easy, according to this invention, to quickly determine how this surface coating will behave under the influence of a certain corrosive liquid. During a test run most of the external conditions can be altered to a great extent. Accordingly the temperature difi'erence across the test specimens can easily be varied by adjusting the outlet with throttle valve 23. To the corrosive liquid it is possible to add abrasives, and other substances, that will bring about erosioncorrosion. Furthermore, the rotation speed and the distance between the rotor and the test specimens can be varied continuously when working. The effects of these alterations can momentarily be studied with the help of the short circuit current curves.

Under some circumstances it is desirable to have cathodic protection in a pump or similar apparatus. The density of the current for this purpose can be calculated from a polarization diagram, or can be estimated, and its protective influence checked in the apparatus according to the invention.

The invention can be utilized in other ways and the described preferred embodiment modified within the scope of the appended claims. Thus, the corrosive liquid can be the sea and the apparatus holding the specimens can be a ships hull, whereby the different water speeds are found due to the curvature of the hull.

The expression chamber should be interpreted in its broadest sense and therefore can comprise while pipe systems and so on. The rotor 10 can be excluded, if the corrosive liquid is given such initial speed and the chamber is so constructed as to give variable liquid speeds across the test specimens. The fact initial speed for the corrosive liquid can be obtained with the help of a pump or a fan.

We claim:

l. A method for performing comparative studies of the corrosion resistance of a material by subjecting it to different flow velocities of corrosive liquid comprising the steps of making a series of test specimens of the material to be tested; mounting said series of test specimens in holes in a body in such a manner that they will be electrically insulated from each other and will each present one exposed surface to said corrosive liquid along one side face of the body; flowing said corrosive liquid along said one side face of the body in such a manner that each individual specimen is subjected to a different corrosive fluid velocity; electrically connecting the series of test specimens to a measuring instrument suitably for reading of galvanic currents; and individually disconnecting in turn each test specimen and reading the galvanic current between the temporarily disconnected specimen and the remaining specimens to obtain the various galvanic current values.

2. An apparatus for performing comparative studies of the corrosion resistance of a material by subjecting it to different flow velocities of corrosive liquid comprising a body having a number of holes in which a series of test specimens of said material, each having a head and a shaft, are mounted electrically insulated from each other and with the outer surface of each head flush with one side face of the body; means for making the corrosive liquid flow along said side face so that each individual test specimen will be subjected to a different liquid flow velocity; conducting means for connecting all the test specimens to a measuring instrument suitable for reading galvanic currents; and further means for individually disconnecting each test specimen from the series so as to make possible a reading of the galvanic current between the temporarily disconnected specimen and the remaining specimens to obtain the various galvanic current values.

3. The apparatus as claimed in claim 2 further comprising a housing of corrosion resistant and dielectric material enclosing a chamber, said housing having an inlet and an outlet for said corrosive liquid, at least one wall of said housing forming said body and wherein said holes for the test specimens are located in such a manner in relation to said inlet and outlet that said corrosive liquid will pass the surface of each of the test specimens in said series with difierent velocities.

4. The apparatus as claimed in claim 3 in which a throttle valve is located in said outlet.

5. The apparatus as claimed in claim 4 and further adapted to study the influence of cavitation upon the material in question wherein a number of bores are formed in the wall of the housing having said hole and one bore is located upstream of each hole relative to the direction of flow of the corrosive liquid, each bore having an axially displaceably matching plug therein.

6. The apparatus as claimed in claim 3 in which the housing is designed as a cylinder provided with flat end walls and a rotor being enclosed in said chamber and spaced from said end walls, said inlet being centrally located in one of said end walls, said outlet being arranged at the periphery of the cylinder, said holes for the test specimens being located in the end wall having said inlet and being located at different radial distances therefrom.

7. The apparatus as claimed in claim 6 in which said outlet from the chamber is designed as a Pitot tube extending from the cylinder wall into the clearance between the end wall and that side of said rotor which is remote from the inlet.

1! I I I 

2. An apparatus for performing comparative studies of the corrosion resistance of a material by subjecting it to different flow velocities of corrosive liquid comprising a body having a number of holes in which a series of test specimens of said material, each having a head and a shaft, are mounted electrically insulated from each other and with the outer surface of each head flush with one side face of the body; means for making the corrosive liquid flow along said side face so that each individual test specimen will be subjected to a different liquid flow velocity; conducting means for connecting all the test specimens to a measuring instrument suitable for reading galvanic currents; and further means for individually disconnecting each test specimen from the series so as to make possible a reading of the galvanic current between the temporarily disconnected specimen and the remaining specimens to obtain the various galvanic current values.
 3. The apparatus as claimed in claim 2 further comprising a housing of corrosion resistant and dielectric material enclosing a chamber, said housing having an inlet and an outlet for said corrosive liquid, at least one wall of said housing forming said body and wherein said holes for the test specimens are located in such a manner in relation to said inlet and outlet that said corrosive liquid will pass the surface of each of the test specimens in said series with different velocities.
 4. The apparatus as claimed in claim 3 in which a throttle valve is located in said outlet.
 5. The apparatus as claimed in claim 4 and further adapted to study the influence of cavitation upon the material in question wherein a number oF bores are formed in the wall of the housing having said hole and one bore is located upstream of each hole relative to the direction of flow of the corrosive liquid, each bore having an axially displaceably matching plug therein.
 6. The apparatus as claimed in claim 3 in which the housing is designed as a cylinder provided with flat end walls and a rotor being enclosed in said chamber and spaced from said end walls, said inlet being centrally located in one of said end walls, said outlet being arranged at the periphery of the cylinder, said holes for the test specimens being located in the end wall having said inlet and being located at different radial distances therefrom.
 7. The apparatus as claimed in claim 6 in which said outlet from the chamber is designed as a Pitot tube extending from the cylinder wall into the clearance between the end wall and that side of said rotor which is remote from the inlet. 